1. Field of the Invention
The invention pertains to a method for controlling a transducer device in level sensors according to the preamble of claim 1, as well as to a device for carrying out this method.
2. Description of the Related Art
Such a method is known, e.g., from German Patent Application No. DE 19 621 449 A1 of the applicant. This document also describes the design in principle of a fork resonator.
A general requirement in oscillating resonators and oscillating fork systems is the monitoring of connector plugs and lines for breaks between the transducer device, which is generally formed as a piezoelectric actuator, and the evaluation electronics.
Until now, this problem has been solved, e.g., by feeding back the transmitted signal over a corresponding second parallel line. If the feedback signal is missing, then there is a defect in the connection, and the sensor electronics produce a fault signal. However, based on the corresponding high technical expense, e.g., required by the doubling of signal lines and plug connector terminals, is well as by the associated evaluation circuit, in practice it must be sufficient to monitor the ground line, which allows only the detection of gross errors, such as a possibly unplugged plug connector or severed cable wire.
Another solution to the problem is described in the initially cited German Patent Application No. DE 19 621 449 A1 of the applicant. Here, a resistor is connected in parallel with the piezoelectric element used for monitoring. The signal change generated in this way is compensated by equal and out-of-phase power supplied to a resistor. If there is an interruption at an arbitrary point of the signal current circuit, then the current balancing point is disturbed and the oscillating frequency of the fork resonator leaves the nominal operating region, which leads to the output of a fault signal.
The problem with this last solution is that for reduced-size oscillating forks with three times the oscillating frequency relative to standard oscillating forks, the parallel resistor must have a value that is three-times smaller in order to effect a sufficient frequency shift in the error case for tripled idle currents. However, the usable signal is excessively damped by such a reduced-size parallel resistor. Furthermore, the mounting of a monitoring element on the actuator is only possible by complicated means due to the extremely narrow spatial requirements for reduced-size oscillating forks. In addition, for high-temperature operation of the oscillating forks, the parallel resistor and its contacts must be suitable for prolonged exposure to such high temperatures (e.g., 200xc2x0 C.).
The task of the invention is to present a method and a device that monitor the entire power circuit of the transducer device for interruptions without requiring additional lines or components on the actuator element itself. In addition, the method or device to be disclosed should prevent signal damping and should guarantee failure security of the entire system.
This task for the method is solved through the features of claim 1.
A device for carrying out the method is the object of claim 11.
Refinements of the invention are the object of the subordinate claims.
The essence of the invention is that during the oscillation excitation, the capacitance between the lines of the transducer device or a variable proportional to it is detected, and if there is deviation from a predetermined desired value by a predetermined amount, then a fault signal is generated. Thus, simultaneously during the oscillating process, the capacitance of the transducer device, e.g., of the piezoelectric element of such a transducer device including the cable supply line is determined and when a predetermined value is not met, a fault signal is produced.
The capacitance measurement is done preferably through evaluation of the current or the amount of current (charge) that flows during discharge of the piezoelectric element and supply line.